There are a wide variety of metal oxides and such material oxides are used for a variety of applications. For example, indium oxide is a well-known material and is used as the material of a transparent electrode needed in a liquid crystal display or the like.
Some metal oxides have semiconductor characteristics. Examples of metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. Thin film transistors in each of which a channel formation region is formed using such a metal oxide have already been known (for example, see Patent Documents 1 to 4, Non-Patent Document 1, and the like).
Further, not only single-component oxides but also multi-component oxides are known as metal oxides. For example, InGaO3(ZnO)m (m: natural number) having a homologous series is known as a multi-component oxide containing In, Ga and Zn (for example, see Non-Patent Documents 2 to 4 and the like).
Furthermore, it is confirmed that an oxide semiconductor including such an In—Ga—Zn-based oxide is applicable to a channel formation region of a thin film transistor (for example, see Patent Document 5, Non-Patent Documents 5 and 6, and the like).